1. Field
The present invention relates to disk drives for computer systems. In particular, the present invention relates to a disk drive writing wedge RRO values in a butterfly pattern.
2. Description of the Related Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk typically comprises a number of concentric data tracks each partitioned into a number of data sectors. Access operations are performed by seeking the head to a target data track, and performing a write/read operation on the data sectors within the data track. The disk typically comprises embedded servo sectors having position information recorded therein, such as coarse position information (e.g., a track address) and fine position information (e.g., servo bursts). A servo controller processes the servo sectors to position the head over the target data track.
The embedded servo sectors may be written to the disk using any suitable technique, such as with an external servo writer, a media writer, or self-servo written. An external servo writer writes the servo sectors by servoing the actuator arm (with a pin) while using the head internal to the disk drive to write the servo sectors, whereas a media writer writes the servo sectors onto a number of disks which are then inserted into production disk drives. Alternatively, each disk drive may self-servo write the servo sectors using any suitable technique, such as propagation or while following seed tracks, such as spiral tracks.
Regardless as to how the servo sectors are written, errors in the servo writing process typically means the servo sectors will define eccentric tracks. In addition, eccentricity may be due to the disk being clamped to the spindle motor after being servo written with a media writer, or if the disk slips after being clamped to the spindle motor. The eccentricity is referred to as repeatable runout (RRO) since the position error relative to the head will repeat each time the disk rotates. The RRO typically comprises a component based on the rotational frequency of the disk (fundamental or 1F frequency) as well as higher frequency components unique to each track.
To compensate for the eccentricity, RRO compensation values are generated and processed by the servo controller to either cancel or follow the eccentricity. RRO values corresponding to the fundamental frequency (and optionally the harmonics) are typically learned and then modeled with a sinusoid. The RRO values that correspond to the higher frequency components are typically generated for each track and then written at the end of each servo sector (referred to as wedge RRO values or WRRO values). As the servo controller reads each servo sector, an RRO compensation is generated using the sinusoid RRO value and the WRRO value.
The WRRO values are typically learned and then written to the disk as part of the disk drive manufacturing process. It is therefore desirable to minimize the time it takes to write the WRRO values to the disk in order to minimize the bottleneck and corresponding manufacturing expense.